1. Technical Field
The present invention relates generally to an improved data processing system and in particular to a data storage subsystem for use with a data processing system. Still more particularly, the present invention provides a dynamically changeable virtual mapping scheme in a data processing system.
2. Description of Related Art
In computer systems and data storage subsystems, one problem is performing a data file copy operation in a manner that minimizes the use of processing resources and data storage memory. Previously, data files were copied in their entirety by the processor, such that two exact copies of the selected data file were resident in the data storage memory. This operation consumed twice the amount of memory for the storage of two identical copies of the data file. Additionally, this operation required the intervention of the processor to effect the copy of the original data file.
A data file snapshot copy is an improvement over this type of copy process. This snapshot copy process includes a dynamically mapped virtual data storage subsystem. This subsystem stores data files received from a processor in back-end data storage devices by mapping the processor assigned data file identifier to a logical address that identifies the physical storage location of the data. This dynamically mapped virtual data storage subsystem performs a copy of a data file by creating a duplicate data file pointer to a data file identifier in a mapping table to reference the original data file. In this dynamically mapped virtual data storage subsystem, the data files are referred to as a collection of “virtual tracks” and each data file is identified by unique virtual track addresses (VTAs). The use of a mapping table provides the opportunity to replace the process of copying the entirety of a data file in the data storage devices with a process that manipulates the contents of the mapping table. A data file appears to have been copied if the name used to identify the original data file and the name used to identify the copy data file are both mapped to the same physical data storage location.
This mechanism enables the processor to access the data file via two virtual track addresses while only a single physical copy of the data file resides on the back-end data storage devices in the data storage subsystem. This process minimizes the time required to execute the copy operation and the amount of memory used since the copy operation is carried out by creating a new pointer to the original data file and does not require any copying of the data file itself.
One implementation of the snapshot copy process provides a two-table approach. One table has table entries for each virtual device track pointing to another table containing the physical track location for the entry. Each physical track table entry identifies the number of virtual track entries that point to this entry by use of a reference count mechanism. Each virtual track entry that points to the physical track is called a “reference.” The reference count increments when a new virtual track table entry pointer points to this physical entry (e.g. snap) and the reference count decrements when a virtual track table entry pointer is removed (e.g. update source after a snap). When a reference count is zero, then that physical track can be deleted from the back-end since it is known that there are no references to the physical track.
System administrators are beginning to realize that “point in time” or “instant” copies of data are extremely useful. However, the system administrator has to specifically plan for and request execution of these copies at the host level, such as setting up mirrored volumes or using the snapshot commands available in virtual mapping subsystems.
In addition, when attempting to provide the benefits of virtualized data storage, some type of mapping scheme is required. One of the problems with some of the existing mapping schemes is the additional processing overhead needed to process the mapping algorithm or following the mapping pointers to find the location of the desired data. Some of the mapping schemes force the manipulation of many pointers in order to perform operations on large sets of mapped data. Some mapping schemes also force the allocation of mapping tables for all possible virtual addresses whether or not those addresses are actually used.
In addition, RAID (redundant array of inexpensive disks) disk subsystems are traditionally organized by a set of disk drives into a RAID group. The RAID group can be viewed as a single logical unit. Furthermore, the capacities of disk drives have been increasing to such a size that operating systems of file systems may not utilize all of the space of a RAID group. In an attempt to resolve this, some RAID products are capable of partitioning a bound drive set into multiple logical units.
In most cases, with RAID products partitioning a bound drive into a set of multiple logical units, the RAID subsystem requires all units to be homogenous. In only a few cases, heterogeneous logical units with similar attributes can be combined in a RAID group. In general, these units need to meet the lowest common denominator of capacity to have a consistent device relative address for RAID stripe allocation.
However, the one exception to this method of associating RAID groups is the HP AutoRAID. The HP AutoRAID has a close analogy to the storage pool invention defined here but is different in concept. In HP AutoRAID, all drives comprise the basis to one of two RAID sets. There is one RAID one and one RAID five set. Drives are partitioned into groups on request. The AutoRAID does not provide a common space capacity. Capacity is managed across all units to satisfy the RAID group requirement.
Therefore, it would be advantageous to have a system using a multi-layer virtual mapping scheme where the mapped units that are involved in managing storage allocation are tracked using a bit map and exception table, thus reducing the table entries to be manipulated to perform storage management operations.